1. Field of the Invention
The present invention relates to a variable transconductance amplifier. Such an amplifier may be used, for example, in radio frequency circuitry as part of an automatic gain control (AGC) circuit. For example, such an amplifier may be used within a mixer or front end low noise amplifier (LNA).
2. Description of the Prior Art
FIG. 1 illustrates a known type of voltage controlled variable gain amplifier comprising NPN bipolar transistors. The amplifier comprises a transconductance stage in the form of transistors T1 and 12, resistors R1 and R2 and a constant current source CC1, one of whose terminals is connected to ground gnd. The bases of the transistors T1 and T2 are connected to differential signal input terminals IN+ and IN-, respectively.
The collectors of the transistors T1 and T2 are connected to a current steering circuit comprising transistors 1 to 4. The emitters of the transistors 1 and 2 are connected to the collector of the transistor T1 whereas the emitters of the transistors 3 and 4 are connected to the collector of the transistor T2. The collectors of the transistors 2 and 4 are connected to a positive supply input or line vcc. The collectors of the transistors 1 and 3 are connected to differential output terminals OUT- and OUT+ and via load resistors 5 and 6, respectively, to the supply line vcc. The bases of the transistors 1 and 3 are connected to a first gain control input "agc adjust+" whereas the bases of the transistors 2 and 4 are connected to a second gain control input "agc adjust". In use, a differential gain control voltage is supplied to the gain control terminals so that, when maximum gain is required, all of the signal current from each of the transistors T1 and T2 is steered through the respective load resistor 5 and 6. Conversely, when minimum gain is required, the signal current is steered via the transistors 2 and 4 to the supply line vcc.
Although a voltage controlled variable gain amplifier of the type shown in FIG. 1 may be used in an AGC arrangement of a radio frequency (RF) tuner, it has certain disadvantages when used in such an application. For example, as the gain of the amplifier decreases, the noise figure (NF) increase. Also, using NPN technology (or NMOS technology for field effect transistors) increases the difficulty of connecting the outputs to, for example, a mixer of a frequency changer.
Further, in order to handle large amplitude input signals, the transconductance input stage requires sufficient power to maintain linearity. This may be achieved by providing the resistors R1 and R2 with relatively high values and using a low quiescent current, in which case the noise figure of the transconductance stage is compromised. Alternatively, higher quiescent current and lower values of the resistors R1 and R2 may be used but this results in increased power dissipation.